The genetic phenomenon that occurs when two different alleles contribute to a phenotype is known as codominance or incomplete dominance, depending on the context. In codominance, both alleles express themselves fully, as seen in is AB blood type, where both A and B antigens are present. In incomplete dominance, a blend of traits manifests, such as in the case of red and white flowers producing pink offspring. Understanding these concepts sheds light on the complexity of inheritance and variation in trait expression.